U.S. Air Force Staff Sgt. Luis A. Walker arrives from a lunch break during his court martial Monday at Lackland Air Force Base in San Antonio. Walker, a former training officer, is charged with illicit sexual contact with 10 female trainees. He is facing 28 counts including rape and is one of 12 instructors under investigation. (ASSOCIATED PRESS / The San Antonio Express-News, Jerry Lara)

SAN ANTONIO (AP) — Fighting back tears, a woman testified Tuesday that a Texas Air Force base instructor facing charges in a widening military sex scandal refused her pleas of no after luring her into his office and then sexually assaulted her on a bed.

The alleged victim said after the attack, Staff Sgt. Luis Walker told her not to tell anybody about what happened.

“I said no several times, in several different ways. He didn’t accept that answer,” the female airman testified, wiping away the tears that eventually came.

She was the first of four alleged victims who took the witness stand on the opening day of testimony at Walker’s court-martial at Lackland Air Force Base in San Antonio. The women told jurors that Walker gained their trust to get them alone in his office or an empty dormitory and forced them into kissing, touching and sex.

They said they didn’t tell anybody at first because they feared being booted from the Air Force. The Associated Press is not naming them because they are alleged sexual assault victims.

Walker is among 12 instructors at the Lackland base who are facing charges or being investigated. His charges — 28 counts including rape, aggravated sexual contact and multiple counts of aggravated sexual assault — are the most serious in the case. He faces up to life in prison and a dishonorable discharge if convicted by a seven-person jury comprised of military personnel.

The first alleged victim who testified told jurors that before assaulting her in his office, Walker had made sexually suggestive comments to her and hugged and kissed her in a stairwell.

While being questioned by Maj. Naomi Dennis, one of Walker’s attorneys, the alleged victim acknowledged she initially denied that anything inappropriate happened with Walker.

“At the time, I was not ready to talk about what had happened to me,” she said.

Another female airman testified that Walker pushed her against a wall one time in his office and put his hand down her pants.

“He told me if I told anyone about it, I would get kicked out with a dishonorable discharge,” she said.

Defense attorneys tried to discredit the women’s allegations, suggesting they have changed their stories multiple times or had romantic feelings for Walker.

Testimony was to resume Wednesday.

During opening statements Tuesday, one of the prosecutors, Maj. Patricia Gruen, described Walker as a “consummate predator” and a “wolf in sheep’s clothing.” She said as a military instructor, he was responsible for helping mold recruits into airmen but abused his authority.

“He used (his power) so he could gain sexual favors,” Gruen said. “He used it as a way to get his way with trainees.”

Dennis told jurors there is no evidence to substantiate the charges. Walker is accused of raping one female recruit and sexually assaulting or having inappropriate sexual or personal contact with nine others. Under military law, the difference between rape and aggravated sexual assault is that physical force has to be used to constitute rape.

“Ten victims, 28 charges. Those are just numbers. Numbers aren’t proof,” Dennis said. “This case requires you to dig deeper to find the truth. Once you start digging, you will find some pretty incredible scenarios.”

Base officials have described Walker’s case as the “cornerstone” of an ongoing investigation. A total of six instructors, including Walker, were charged.

Staff Sgt. Kwinton Estacio had his case referred to a court-martial Tuesday. No date was set.

In June, Staff Sgt. Peter Vega-Maldonado admitted that he had sex with a female recruit. The base instructor was sentenced to 90 days of confinement as part of a plea deal. He later acknowledged he had been involved with nine other trainees, which prosecutors didn’t previously know about. Prosecutors are not seeking charges in the other cases.

The alleged sexual misconduct at the base apparently began in 2009, but the first alleged victim didn’t come forward until last year.

Investigators have identified at least 31 female trainees believed to be victims in the scandal. A two-star general, Maj. Gen. Margaret H. Woodward, has launched a separate, independent probe, and nearly 80 members of Congress have called for a hearing.

Walker trained recruits for about 18 months, until he was removed from his position in June 2011. He joined the Air Force in 2004 and previously was stationed at Dover Air Force Base in Delaware and at facilities in Montana and Korea. The Air Force is withholding his age and hometown.

Lackland is where every American airman reports for basic training — about 35,000 a year. About one in five is female, pushed through eight weeks of basic training by a group of instructors, 90 percent of whom are men.